This invention pertains to means for the separation of entrained liquids, e.g. aerosols, from gases. By the term "entrained liquids" is meant moisture droplets, oil droplets and the like which are borne as discrete physical particles entrained in a gas as aerosols therein, for example an oil fog.
Various types of apparatus for separating entrained liquids from a gas stream are known in the prior art, directed to particular "de-aeration" applications such as for removing/separating excess entrained lubricating/cooling oil from the compressed air stream output by an air compressor before the compressed air is supplied to pneumatic machines, or for removing/separating moisture particles from the inlet air stream of a marine gas turbine, or for the separation and/or removal of entrained liquid microdroplets from a process gas stream. Examples of various known gas-entrained liquid separation devices are disclosed in U.S. Pat. Nos. 1,552,903; 2,432,130; 3,494,110; 3,548,569; 3,870,493; 4,086,070; 4,092,137; 4,158,449; 4,255,099; 4,300,918; 4,506,523; 4,548,569; and 4,668,256, the disclosures of which are hereby incorporated by reference hereinto.
As an adjunct to separating the entrained liquid aerosols from the gas stream, it is often desirable that the separated liquid be removed, e.g. for collection and/or reuse. Typically, it is desirable for efficient separation that the liquid, once separated from the gas stream, not be re-entrained as an aerosol in the flow of the gas. As a further adjunct, it is often desirable to clean the gas stream.
Other considerations involved in the separation of entrained liquids from gas streams include the factors of separation efficiency, gas flow velocity, permissible pressure drop, and aerosol particulate size and type. These factors are typically interrelated and may often be interdependent, with one or more factors being critical or paramount and thus dictating the particular priority of design considerations. However, because some factors may conflict with others, the design considerations often require compromises such that some aspects of performance must be traded off in favor of others, and thus the performance is less than ideal. Where many factors must be optimized, the separator design can become complex and costly to implement.
The present invention is directed particularly, but by no means exclusively, to the separation and removal of entrained oil droplets ("oil fog") from an oil-laden air stream venting from a sump bearing of a gas turbine. A significant concern in such an application is compliance with regulations proscribing standards governing the venting/emission of aerosols such as oil fog into the atmosphere. By way of example, it is desired to prevent entrained oil fog from being vented directly into the atmosphere. For compliance with applicable governing standards, it thus becomes desirable to remove sufficient oil fog from a vented oil-laden sump bearing air stream to reduce the residual oil-in-air concentration in the air stream vented to the atmosphere to below the "visible" level, for example below 30 PPM (parts per million).
However, the provision of an apparatus directed to achieving such a level of separation performance must necessarily take into consideration also the factors of air flow volume and velocity, temperature, and tolerable air stream pressure drop, as well as the oil content in the vent air stream, which last factor may not be predictable and which may vary considerably over time. Further, some factors may vary considerably depending upon the turbine operating conditions. Another consideration involves the fact that the entrained oil fog may typically consist of many different sizes of oil droplets.
By the present invention, an apparatus is provided for separating and removing entrained oil droplets from a gas stream, which apparatus includes a preseparator/separator combination which is arranged in a housing means having in a lower part thereof an annular inlet chamber means for admitting and subjecting an entrained oil droplet-laden air stream to centrifugal action, direction reversal and velocity change, for forcing the larger oil droplets out of the air stream by centrifugal force acting thereon and causing the larger oil droplets to impinge on an impingement surface, and then coalescing the impinged oil droplets into a liquid and draining off this separated oil to a sump in the lower part of the housing means.
Then the air stream, still transporting entrained oil droplets, is next conducted to pass through a preseparator means also arranged in the lower part of the housing means and containing an impingement and coalescence medium for causing impingement and coalescence of the transported oil droplets. The preseparator also includes means for collecting the coalesced oil and draining it to the sump in the lower part of the housing means.
After passing through the preseparator means the air stream may still contain residual oil microdroplets. The apparatus further includes a principal separator means arranged in an upper part of the housing means and communicating with the preseparator means. The air stream exiting the preseparator means and still containing residual microdroplets is conducted upward to pass through the principal separator means. The principal separator means is of two stage configuration and includes an upstream stage separator element and downstream stage separator element. The upstream stage separator element promotes coalescence of the residual microdroplets into liquid. The downstream stage separator element prevents, by impingement, the re-entrainment of any now relatively large coalesced oil droplets which may emerge from the upstream stage separator element, and also directs drainage of the coalesced oil to a sump provided in the upper part of the housing means. The air stream, now purged of entrained oil to an acceptable level, is then conducted to flow from an outlet in the upper part of the housing means to be released into the atmosphere.
The sumps in the lower and upper parts of the housing means are provided with drains to be connected to a receiving sump for receiving the separated liquid oil.